The present invention relates to focused cesium ion beams. More specifically, the invention relates to focused cesium ion beams that are useful in analyzing a microarea in the field of material analysis by being employed in secondary ion mass spectrometers for material analysis.
In the field of semiconductors, particularly the microminiturization of semiconductor devices and large scale integration, slight contamination and introduction of micro particles during the process of producing semiconductor devices may affect the operation of the devices and inflict fatal defects in some cases. As cleanliness in the process of producing semiconductor devices is noticeably improved, the amount of contaminants that can be tolerated is reduced, so that ever smaller particles are capable of producing defects. Consequently, high sensitivity and high spatial resolution have become required of apparatus for detecting and analyzing these contaminations and small particles. Secondary ion mass spectroscopy (hereinafter referred to as SIMS) has an intrinsic property than it has a higher sensitivity that surface analyzers and it can make an analysis in the depth direction, so that it is much desired for trace/micropart analysis.
Since cesium (C.sub.s) used in SIMS increases the probability of secondary ionization of an element having high ionization potential, such as C, O, H, Au, and Ag, the use of cesium in some forms is well known to be indispensable to the SIMS. A surface ionization C.sub.s ion source, C.sub.s vapor source and the like have been employed for introducing C.sub.s onto a sample surface.
The surface ionization C.sub.s ion source has a low spatial resolution on the analysis area, because the diameter of the ion beam is large. Therefore it is not suitable for trace/micropart analysis, because the low spatial resolution produces a small sample current density.
The C.sub.s vapor source, has a problem that areas other than the analysis area are contaminated by C.sub.s. This results in a lack of reliability in the depth direction analysis when a plurality of adjoining areas are analyzed.
As a result, the methods using the surface ionization C.sub.s ion source and C.sub.s vapor source of C.sub.s have not been most preferred when working with trace/micropart analysis or analysis of adjoining areas.
Attention is now being directed to focused ion beams (hereinafter called FIB) for irradiating a submicron area of a sample with high current density ions. The diameter of a FIB is narrowed by focusing to approximately 0.1 .mu.m on the surface of the sample irradiated. A liquid metal ion source (hereinafter called LMIS) is one of the ion sources for implementing FIB formation. With a LMIS, a high electric field is applied to a needle-shaped emitter, i.e. an emitter having a sharp tip, after the emitter is wetted with liquid ion material. The liquid ion material is thus ionized and emitted by both field evaporation and field ionization. The effectiveness of the LMIS is attributed to the focusing capability of the emitted ion beam that can be focused to approximately 0.1 .mu.m in diameter, the current density that is higher by several orders of magnitude than that of the beam obtained by the surface ionization source, and the numerous ionic species that can be emitted.
An application of the LMIS technology to a SIMS has already been attempted by using gallium (Ga), which material is easy to handle as an ion material. Microarea analysis has also been carried out using a Ga.sup.+ FIB. For instance, R. Levi Setti et al reported such an analysis under the title, "Aspects of High Resolution Imaging With a Scanning Ion Microprobe" (Ultramicroscopy, 24) (1989) pages 97-113. Although the microarea analysis was realized by the application of the Ga.sup.+ FIB to SIMS, the shortcoming is that the sensitivity to light elements such as C, H and O is low.
S. P. Thompson discloses an example of mounting a liquid metal cesium ion source (hereinafter called C.sub.s LMIS) on a SIMS in the paper entitled, "Artifact In High Resolution SIMS: The Contribution of the Ion Source" (Vacuum, 34) (1984) pages 947-951. However, the ion beam diameter in this case is still as large as 100 micrometers and no results of submicron area analysis have been obtained. Further, not even the formation of a cesium ion beam of submicron diameter has not been attained.
It is known that a total ion current (I.sub.T) range of a C.sub.s LMIS for providing the highest current density can be determined by experimentally passing ion beams through a focusing optical system by adjusting the operational characteristics of LMIS, after first estimating characteristics such as angular intensity and full width at half maximum energy.